It is generally accepted that blades and vanes in the turbine section of modern gas turbine engines operate in one of the most severe environments in the engine. As a result, various processes and alloy compositions have specifically been developed to fabricate such components.
Today, turbine section blades and vanes are typically fabricated by investment casting techniques; three techniques have been found to be particularly useful, and each produces a particular grain structure. The structure produced using conventional casting techniques is characterized by generally equiaxed, randomly oriented grains. The two other useful investment casting techniques are types of directional solidification (DS); in one such technique, the cast component is characterized by a multiplicity of substantially parallel (columnar) grains. In the other technique, the cast component is a single crystal, i.e., contains only one grain.
Components produced using conventional casting techniques have mechanical properties which are generally equivalent irrespective of the direction in which they are measured. The mechanical properties of DS components are, however, dependent upon the direction in which they are measured, i.e., these components are anisotropic. For example, the room temperature modulus of elasticity (tensile stress per unit strain in the elastic regime) for directionally solidified nickel base superalloys of the type typically used in gas turbine engines is about 18.times.10.sup.6 psi in the &lt;100&gt; direction; about 33.times.10.sup.6 psi in the &lt;110&gt; direction; and about 46.times.10.sup.6 in the &lt;111&gt; direction.
Due to recent trends towards very high gas stream temperatures in the turbine section, resistance to thermal fatigue cracking has become the life limiting property for some single crystal airfoils (blades and vanes). Because a low modulus of elasticity is generally indicative of good thermal fatigue cracking resistance, single crystal castings are fabricated such that the low modulus [001] axis is substantially aligned with the primary stress axis of the part. In general, the primary stress axis lies on the longitudinal axis of the part. According to U.S. Pat. No. 3,494,709, the angle between the [001] crystal axis and the longitudinal axis of a gas turbine engine airfoil should be less than 20.degree.. In some currently used single crystal airfoils, this angle is no greater than 15.degree..
As noted above, specific alloy compositions have been developed for turbine components such that castings made of the alloys exhibit desirable properties. However, alloys which have useful properties when conventionally cast or when cast into columnar grain form may not have equally useful properties when cast into single crystal form. This is generally attributed to the fact that articles having a conventional or columnar grain microstructure generally require grain boundary strengthening elements for the necessary strength at high temperatures. Such elements include carbon, boron, zirconium, and hafnium, as described in U.S. Pat. Nos. 3,567,526, 3,700,433, 3,711,337, 3,832,167, 4,078,951 and 4,169,742. Single crystal articles have no internal grain boundaries, and as discussed in U.S. Pat. Nos. 3,494,709 and 3,567,526, the presence of C, B, Zr, or Hf should be avoided. These patents teach that boron and zirconium impair the properties of single crystals, and that carbon, if present at all, should be limited to 100 parts per million (ppm) in the alloy composition. U.S. Pat. No, 4,209,348 describes a single crystal alloy composition in which no intentional additions of C, B, Zr, or Hf are made, although such elements may be present as impurities. The individual C, B, Zr, and Hf levels should be less than 50 ppm, and the combined C+B+Zr+Hf content should be less than 100 ppm.
U.S. Pat. No. 4,459,160 describes single crystal alloy compositions wherein the C+B+Zr+Hf content may exceed 100 ppm, although no intentional additions of B, Zr or Hf are made. U.S. Pat. No. 4,488,915 describes a single crystal composition which permits no zirconium or boron, but up to 500 ppm carbon. However, it is stated that most of the carbon will transform to carbon monoxide during the casting process, and as a result, the composition of the solidified article will actually contain only a little carbon.
U.S. Pat. No. 4,402,772 describes a single crystal composition which has good oxidation resistance, apparently due, in part, to the presence of about 500 ppm hafnium.
While numerous compositions for single crystal articles exist, they all suffer from anisotropy, which limits their utility. Accordingly, engineers continue to work to develop improved alloy systems, particularly those in which castings made of the alloys have reduced anisotropy. Such alloy compositions would permit, e.g., the use of articles wherein the [001] crystal axis is more than 20.degree. from the primary stress axis of the article.